Insights of the anionic redox in P2–Na0.67Ni0.33Mn0.67O2

The search for high-energy batteries has promoted intense attention to anionic redox in layered transition metal oxides because of their ability of delivering much higher capacity than traditional cathodes. P2–Na0.67Ni0.33Mn0.67O2 electrode exhibits outstanding air-stability and high average potential. Very recently, the anionic redox in this promising sodium cathode has been evidenced by mapping of resonant inelastic X-ray scattering. However, the origin of this oxygen redox has not been recognized yet. Here, based on the combination of X-ray absorption spectroscopy and density functional theory (DFT) calculations, we demonstrate that with the remove of Na+, the Ni2+ oxidized to Ni3+ and followed by the oxidation of lattice oxygen. Our DFT calculation further confirms that the oxygen redox in Na0.67Ni0.33Mn0.67O2 is rooted from Ni–O anti-bonding (eg*) state rather than the non-bonding O2p band and result in the highly reversible oxygen redox reactions without O2 loss. Moreover, at very low sodium contents, it is highly possible that a charge redistribution process between the Ni and O ions occurs, which results in the inconsistent experimental observations in previous references.